Microsoft tests fix for critical IE bug

'Fix it' tool available for browser flaw

Microsoft is testing a patch for a critical vulnerability in Internet Explorer (IE), but has stopped short of promising to deliver an emergency fix before the next scheduled Patch Tuesday.

"We have seen speculation that Microsoft might release an update for this issue out-of-band," said Jerry Bryant, a senior manager with the Microsoft Security Response Center (MSRC), referring to the term the company uses for a rush fix. "I can tell you that we are working hard to produce an update which is now in testing."

Microsoft first warned users of the vulnerability in IE6 and IE7 last week when it said hackers were exploiting the bug. The next day, Israeli security researcher Moshe Ben Abu grabbed the attack code from a site that had been using it to conduct 'drive-by' attacks, then crafted a public exploit, which he published to the popular Metasploit penetration testing framework.

Even before Abu posted his exploit, security experts had said they expected Microsoft to ship an out-of-band update if attack code went public.

Bryant, however, did not commit Microsoft to an emergency patch, saying only, "We never rule out the possibility of an out-of-band update," in an entry on the MSRC blog Friday. Testing, he added, was a "critical and time intensive" part of the process because Microsoft had to confirm the update worked with all versions of both IE and Windows. Only IE6 and IE7 harbour the bug; the oldest and the newest editions, IE 5.01 and IE8 respectively, do not.

Microsoft is also now offering an automated 'Fix it' tool to disable the component in the 'iepeers.dll' file that contains the vulnerability. The free tool works on machines powered by Windows XP or Windows Server 2003. That workaround was an addition to those that Microsoft recommended last Tuesday, which included disabling scripting, enabling DEP (data execution prevention) and upgrading to IE8 .

Rival browsers, including Mozilla's Firefox, Google's Chrome and Opera Software's Opera, are also safe from the in-the-wild attacks aimed at IE6 and IE7.

The newest zero-day is the second this year that Microsoft has admitted hackers have exploited before a patch was ready. In mid-January, Microsoft said that a flaw in IE had been used to attack several companies' networks, including Google's and Adobe's. Microsoft patched the vulnerability on January 21 in an out-of-band update.

Microsoft's next scheduled Patch Tuesday is April 13, more than four weeks away.